1. Field of the Invention
This invention relates to a data carrier with a printed image produced by intaglio, to an intaglio printing process as well as to printing plates for carrying out the intaglio printing process and a method for producing the printing plates.
2. Related Art
It is distinctive for gravure printing that the printing, i.e. ink-transferring, areas of the printing plate are present as depressions in the plate surface. These depressions are produced by a suitable engraving tool or by etching. Before the actual printing operation, ink is applied to the engraved plate and surplus ink removed from the surface of the plate by a stripping doctor blade or wiping cylinder so that ink is left behind only in the depressions. Then a substrate, normally paper, is pressed against the plate and removed, the ink adhering to the substrate surface and forming a printed image there. If transparent inks are used, the thickness of inking determines the color tone. The high bearing pressure subjects the substrate material additionally to embossing, which also stands out on the back of the substrate.
Among gravure printing techniques a distinction is made between rotogravure and intaglio or line intaglio. In rotogravure, the printing plates are produced for example by electron beam, laser beam or graver. It is distinctive for rotogravure that different gray or color values of the printed image are produced by cells of different density, size and/or depth disposed regularly in the printing plate.
In contrast, in intaglio linear depressions are formed in the printing plates to produce a printed image. In the mechanically fabricated plate for intaglio, a wider line is produced with increasing engraving depth due to the usually tapered engraving tools. Furthermore, the ink receptivity of the engraved line and thus the opacity of the printed line increases with increasing engraving depth. In the etching of intaglio plates, the nonprinting areas of the plate are covered with a chemically inert lacquer. Subsequent etching produces the engraving in the exposed plate surface, the depth of the engraved lines depending in particular on etching time and line width.
The intaglio technique, in particular the steel intaglio technique, provides a characteristic printed image that is easily recognizable to laymen and not reproducible with other common printing processes. If the engravings in the printing plate are deep enough, a data carrier printed by intaglio is given through embossing and inking a printed image that forms a relief perceptible with the sense of touch. Steel intaglio is therefore preferably used for printing data carriers, in particular security documents and documents of value, for example bank notes, shares, bonds, certificates, vouchers, security labels and the like, which must meet high standards with respect to forgery-proofness.
WO 97/48555 discloses a method for producing intaglio printing plates in reproducible fashion by machine. The lines of a line original are detected and the surface of each line precisely determined. An engraving tool, for example a rotating graver or laser beam, is first used to engrave the outside contour of this surface to cleanly border the surface. The bordered area of the surface is then cleared by means of the same or another engraving tool so that the total line is precisely engraved in accordance with the line original. Depending on the form and guidance of the engraving tool, a basic roughness pattern serving as an ink trap for the printing ink arises at the base of the cleared surface.
To obtain a sufficient measure of tactility of the image produced by intaglio, very thick inking is required according to prior art methods. Thick inking, however, at the same time means high consumption of ink, which in turn results in high production costs. In addition, the quantity of ink applied to the plate must be increased in the conventional technology to close with ink all engraved areas that are to yield tactile structures in the printed image. Thus, there is more surplus ink that must be removed from the surface of the plate by a stripping doctor blade or wiping cylinder, which leads to problems in the waste disposal of the wiped inks.